The present invention relates to a plasma display apparatus that provides a gradated display using a subfield method.
The plasma display apparatus (PDP apparatus) has been put into practical use as a flat display and is a thin display of high-luminance. In the PDP apparatus, since it is only possible to control each display cell to be lit or not, a display field is made to consist of plural subfields and the subfields to be lit are combined in each cell to provide a gradated display. Each subfield comprises at least an address period during which a display cell is selected and a sustain period during which the selected cell is lit. In the sustain period, a sustain pulse is applied to cause a sustain discharge to occur, and the luminance is determined by the number of sustain pulses. As a result, if the cycle of the sustain pulse is the same, the luminance is determined by the length of the sustain period. Although the most general and efficient structure of the subfield is that in which the lengths of the sustain periods in the subfields, serially increase and the ratio of the length of the sustain period in a subfield to that of the previous one is 2, various subfield structures have been proposed recently in order to suppress false contours. The present invention can be applied to any PDP apparatus that performs display using any subfield structure.
Moreover, various methods have been proposed for the PDP apparatus, and the present invention can be applied to a PDP apparatus that employs any method. As the structures and the driving methods of the PDP apparatus are widely known, a detailed description is omitted here.
In the PDP apparatus, when the ratio of the cells to be lit to all the cells in the whole screen (display load ratio) is large, a large sustain current flows as a result, and the luminance is degraded because the effective voltage of the sustain pulse is lowered. When the gradated display is performed by the subfield method, a problem is caused that a normal gradated display cannot be performed because the display load ratio differs from subfield to subfield and the luminance ratio of each subfield deviates from a specified relationship. In order to solve the problem, Japanese Unexamined Patent Publication (Kokai) No. 9-185343 has disclosed the structure in which the number of sustain pulses in each subfield is corrected to maintain the luminance ratio by detecting the display load ratio in each subfield.
It is one of the problems relating to the PDP apparatus that the peak luminance is inferior to that of a CRT and the power consumption is large. The power control, therefore, is carried in such a way as to display an image of a lower luminance in total by decreasing the number of sustain pulses in each subfield when the luminance of the entire image is high, and to display an image of a higher luminance in total by increasing the number of sustain pulses in each subfield when the luminance of the entire image is low. As a method of controlling power, Japanese Unexamined Patent Publication (Kokai) No. 2000-322025 has disclosed the method in which the cycle of the sustain pulse is shortened when the luminance level is below a specified value by detecting the average luminance level of the entire screen. By using this method, the peak luminance when an image is dark in total can be improved.
When the cycle of the sustain pulse is shortened, the influence of the distortion of the sustain pulse waveform becomes comparably large and it may happen that the specified sustain voltage is not applied. Particularly, when the display load ratio becomes large, the sustain current increases, and the effective voltage to be actually applied is lowered in accordance with the drop in voltage. FIG.1 is a diagram that shows the relationship between the display load ratio and the effective sustain voltage when a pulse of a specified voltage is applied in accordance with the display load ratio for the sustain pulse cycles 6 xcexcS, 8 xcexcS, and 10 xcexcS. If the effective voltage drops, a problem occurs in that the sustain discharge is not caused to occur or the discharge is terminated on the way, resulting in the generation of missing points, or light emission to achieve a normal luminance is not carried out. In the structure disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-322025, the sustain pulse cycle is shortened when the luminance level is low, that is, the display load ratio is small, and the control shown by a short dashed line A in FIG.1 is carried out as a result.
The actual problem, however, is the display load ratio in each subfield when the gradated expression is performed by the subfield method. For example, when the display load ratio in a subfield with a large luminance ratio is very small but that in a subfield with a small luminance ratio is large, the average luminance level (display load ratio) of the entire screen becomes small, and the sustain pulse cycle needs to be shortened according to Japanese Unexamined Patent Publication (Kokai) No. 2000-322025. As a result, the sustain pulse cycle is shortened even in the subfield that has a large display load ratio but a small luminance ratio, and a problem occurs in that such as missing points are generated.
The objective of the present invention is to realize a PDP apparatus in which degradated image such as the generation of missing display points is not caused even though the peak luminance is increased.
In order to realize the above-mentioned objective, the display load ratio of each subfield is detected and the sustain pulse cycle is changed according to the display load ratio of each subfield in the PDP apparatus of the present invention. If, however, the sustain period of each subfield is fixed, the luminance ratio is changed as a result when the sustain pulse cycle of partial subfields is changed. In the present invention, therefore, an adaptive sustain pulse number changing means is provided to increase/decrease the number of sustain pulses in each subfield according to the total amount of variations in time, which is obtained by summing each variation in time caused by the change in the sustain pulse cycle in a display field.
FIG. 2 is a diagram that illustrates the principles of the present invention. As shown schematically, a display field is composed of four subfields SF1 to SF4. Before the sustain pulse cycle is changed, the sustain pulse cycle of every subfield is 8 xcexcS, the sustain periods of SF1 to SF4 are, 80 xcexcS, 160 xcexcS, 320 xcexcS, and 640 xcexcS, and the numbers of sustain pulses of SF1 to SF4 are 10, 20, 40, and 80.
When the display load ratios of SF3 and SF4 are below a specified value, the sustain pulse cycles are changed to 6 xcexcS. In this case, if the duty ratio is fixed, the pulse width will change with the same ratio. If the numbers of sustain pulses of SF3 and SF4 are maintained to 40 and 80, vacant periods of 80 xcexcS and 160 xcexcS are generated in SF3 and SF4, respectively, as a result. Then, with the sustain pulse cycles of SF1 and SF2 being maintained at 8 xcexcS and those of SF3 and SF4 being maintained at 6 xcexcS, the numbers of sustain pulses in SF1 to SF4 are adjusted to 12, 24, 48, and 96, respectively. In this way, the total number of sustain pulses increases from 150 to 180, resulting in the improvement of the peak luminance, while the luminance ratio of each subfield is maintained in the specified relationship. In order to increase the number of sustain pulses in each subfield while maintaining the luminance ratio of each subfield, a vacant time of 96 xcexcS or longer is required, but the vacant time of 48 xcexcS shown schematically is less than the required time and it remains a vacant period. The sustain pulse cycles of SF1 and SF2, the display load ratio of which is large, remain 8 xcexcS, resulting in the generation of no missing points, and although the sustain cycles of SF3 and SF4 become 6 xcexcS, no missing point is generated similarly because of a low display load ratio.
It is also possible to make the sustain discharge stable by, on the contrary expanding the sustain pulse cycle of a subfield when the display load ratio is larger than the specified value. Particularly in the PDP apparatus, the control of power consumption is generally carried out and the total number of sustain pulses is reduced because the power consumption becomes too much when the total number of light emission pulses increases. In this case, a vacant time is generated in a frame, as a result. In this case, therefore it is preferable to make the sustain discharge stable by expanding the sustain pulse cycle. The sustain pulse cycle changing means, therefore, shortens the sustain pulse cycle of each subfield if the display load ratio is lower than the specified value and expands it when higher than the specified value. Although it is possible to treat all the subfields as an object of the frequency modification, it is also possible to treat only partial subfields, that include the one with the maximum luminance, as an object.
The adaptive sustain pulse number changing means increases/decreases the number of sustain pulses so as to maintain the luminance ratio of each subfield.
In addition, as the effective sustain voltage changes and the luminance changes if the sustain pulse cycle is changed, as shown in FIG. 1, it is preferable that an additional adaptive luminance correcting means is provided to correct the change in the luminance due to the change of the sustain pulse cycle, and that the adaptive sustain pulse number changing means increases/decreases the number of sustain pulses of each subfield according to the corrected result.
Moreover, the effective sustain voltage changes depending on the display load ratio of each subfield, therefore, it is preferable to correct the change accordingly and the adaptive sustain pulse number changing means increases/decreases the number of sustain pulses of each subfield.
When the sustain pulse cycle is changed, a large change in display is caused if the cycle is changed considerably, therefore, it is preferable that a change is carried out step by step over plural display subfields so that such a change is not noticed. Moreover, it is preferable that a change is carried out step by step over plural display subfields when the sustain pulse is changed according to the change of the sustain pulse cycle.
When the display load ratio of all the subfields or those that have a specified or higher luminance is lower than a specified value, the control will be easier if the sustain pulse cycle of all the subfields or part of subfields that include the one with the maximum luminance is made identical to each another.